1. Field of the Invention
The present invention relates to an improved wire feed mechanism and more particularly to an improved automatic wire feed system for automatic fine wire bonders.
2. Description of the Prior Art
Automatic wire bonders are classified in Class 228, subclasses 1 to 4.5. Wire feed mechanisms for fine wires used by such wire bonders are classified in Class H 01 L subclasses 21/60 and 23/48.
Manual wire bonder manufacturers recognized the problems associated with wire feeds and were successful in providing fine wire from a large spool by enclosing the wire and wire spool in a closed chamber and feeding the wire through a capillary tube with air pressure as shown in U.S. Pat. No. 3,018,975 issued to Kulicke and Soffa Industries, Inc. Such prior art continuous wire feed mechanism were not provided with means for detecting when a wire bond had not been made or when a break had occurred in the fine wire at the bonding tool.
Fine wire for wire bonding is die drawn to size and rolled onto spools. As the fine wire is unwound from the spool, it often has a spring action or a kink which can pull the fine wire out of the wire guide of a bonding tool when the wire clamps are open or can cause too much wire to be fed through the wire guide of the bonding tool. The amount or size of the wire tail at first bond affects the size of the ball or tail being bonded. The pull or push on the wire also affects the size of the wire loop interconnecting the first and second bond. After the second bond is made and the fine wire is broken, any retraction of the fine wire from the wire guide of the bonding tool can result in a lost wire.
Heretofore, fine wires have been automaticallY capstan fed from a spool to maintain a constant wire slack or loop between the spool and the wire bonding tool as shown in Japanese patent application publication P58-116742. Such wire feed mechanisms are provided with tensioning devices and pinch capstans to control the amount of wire in the loop.
Heretofore, fine wire detectors employed an air blast on a portion of a fine wire to impart either a force or a tension on the wire between the wire spool and the wire bonding tool. Such back tensioning devices are shown in Japanese application publications 57-17141 and 59-115533. Both of these prior art structures maintained a pull or tension on the wire during the bonding cycle which could cause the wire to be pulled from the bonding tool if the wire clamp are open.
If the first bond does not stick or bond properly and the wire clamps are opened when the bonding tool is moved to a second bond position, the pull or tension on the wire at the bonding tool can cause the fine wire to be pulled out of the wire guide. When this condition occurs with an automatic wire bonder, additional maintenance time is required to rethread the fine wire through the wire feed systems.
It is desirable to provide an automatic fine wire feed system which has none of the disadvantages of the prior art systems and places neither tension nor compression in the fine wire leading to the bonding tool and eliminates the effects of spring tension, twisting and kinking.